In recent years, chemical mechanical polishing (or chemical mechanical planarization; hereafter referred to as “CMP”) techniques have been used as global flattening techniques for the surfaces of semiconductor devices, etc. CMP is a process in which indentations and projections in the surfaces of wafers are removed by using a chemical action (dissolution by means of a polishing agent or solvent) together with physical polishing. A polishing apparatus which performs polishing by CMP is equipped with a polishing body and a holding part that holds the object of polishing; the above-mentioned object of polishing is polished by applying a load between the above-mentioned polishing body and the above-mentioned object of polishing, and causing relative motion between the above-mentioned polishing body and the above-mentioned object of polishing, in a state in which a polishing agent is interposed between the above-mentioned polishing body and the above-mentioned object of polishing.
In CMP techniques, monitoring of the polishing state during the polishing process (in-situ monitoring; i.e., detection of the amount of polishing, film thickness or polishing endpoint, etc.) is a problem; there is a great need for such monitoring in order to make the process more efficient.
Recently, monitoring of the polishing state (in-situ endpoint determination and in-situ film thickness measurement, etc.) using optical measurements, i.e., the measurement of reflected light without spectroscopic analysis, or spectroscopic reflection measurements, has been considered effective (Japanese Patent No. 2561812, Japanese Patent Application Kokai No. H11-33901, etc.). In the case of polishing state monitoring devices that perform monitoring of the polishing state by means of such optical measurements, the wafer that constitutes the object of polishing is illuminated with measuring light during CMP, and the film thickness, amount of polishing or polishing endpoint is detected during polishing according to variations in the reflectivity or variations in the spectroscopic reflectivity.
Conventionally, furthermore, even when the polishing of a certain wafer has been completed, the above-mentioned measuring light is supplied to the next wafer that is to be polished, so that illumination with a fixed quantity of light is constantly continued. Accordingly, even after the completion of the polishing of a wafer has been detected, this wafer continues to be illuminated by a fixed quantity of the above-mentioned measuring light until the wafer is moved from the polishing position (the position where the wafer is illuminated with the above-mentioned measuring light).
Meanwhile, for example, in a polishing state monitoring device that monitors the polishing state by spectroscopic reflection measurements, it is conceivable that the acquisition of a reference signal and the measurement of dark noise might be performed prior to the polishing of the object of polishing such as a wafer in order to improve the monitoring precision of the polishing state (although this is not seen in the prior art). It is conceivable that the acquisition of a reference signal might be accomplished by conveying a reference mirror which has the same size as the wafer, etc., and which has specified reflection characteristics to the polishing position by means of (for example) the conveying apparatus that conveys the wafer, etc., to the polishing position, and by illuminating this reference mirror with the measuring light. If such a reference signal is acquired beforehand, the monitoring precision of the polishing state can be improved by comparing this reference signal with the measurement signal obtained by illuminating the object of polishing with the measuring light. Furthermore, it is conceivable that the measurement of dark noise might be accomplished by conveying a dark noise measuring member that has the same size as the wafer, etc., and that shows more or less complete absorption of light to the polishing position by means of (for example) the conveying apparatus that conveys the wafer, etc., to the polishing position, and by illuminating this dark noise measuring member with the measuring light. The signal obtained from the detector in this state indicates noise (dark noise) consisting of both noise caused by stray light in the optical system (flare noise, etc.) and noise of the electrical system such as the detector. If dark noise is thus measured beforehand, the monitoring precision of the polishing state is improved by subtracting the dark noise component from the measurement signal obtained by illuminating the object of polishing with the measuring light, and thus removing the noise component. Furthermore, such acquisition of a reference signal and measurement of dark noise are performed periodically at an appropriate frequency.
However, it has been ascertained that if the wafer continues to be illuminated with measuring light following polishing, there is a danger that problems will occur, for example, in the case of polishing of process wafers that have Cu. Here, an example to be considered is a case in which a so-called damascene is formed by polishing the Cu film in a process wafer which is in a state in which devices such as transistors that have pn junctions and inter-layer insulating films consisting of SiO2 are successively formed, and a Cu film is formed over the entire surface on top of these parts. Even if such a process wafer is illuminated with light, the light will be blocked by the Cu film that covers the entire surface if this illumination is performed prior to the polishing of the Cu film, so that there is no problem. As the polishing of the Cu film progresses, the Cu film gradually becomes thinner, so that eventually the Cu film other than the portions of the inter-layer insulating film inside the holes are removed, thus forming a damascene, after which the polishing is ended. When the Cu film other than the portions of the inter-layer insulating film inside the holes is thus removed, light from the outside reaches the pn junction via the exposed inter-layer insulating film, so that a photo-electromotive force is generated in the pn junction, thus leading to the danger that the Cu will be corroded (oxidized) by the photoelectric cell effect.
Furthermore, as was described above, it is conceivable that the monitoring precision of the polishing state might be improved by acquiring a reference signal and measuring the dark noise. In such a case, a reference mirror or dark noise measuring member that has the same size as the wafer, etc., may be conveyed to the polishing position instead of the object of polishing (such as a wafer) using the conveying apparatus that is used for the wafer, etc. In this case, however, the conveying of the object of polishing to or from the polishing position and the acquisition of a reference signal or measurement of dark noise cannot be performed in parallel; accordingly, there is a danger that the polishing throughput will drop.